Capacitor Discharge: Calculating Rate of Voltage Change and Time

[postman]

Homework Statement

When a condenser discharges electricity, the instantaneous rate of change of the voltage is proportional to the voltage in the condenser.
Suppose you have a discharging condenser and the instantaneous rate of change of the voltage is 1/100 of the voltage (in volts per second). How many seconds does it take for the voltage to decrease by 90%?

Homework Equations

e^-rt

The Attempt at a Solution

Log 90= e^-t ? = 4.49?

 

[willem2]

It's wrong. Try to give a more detailed calculation and explain what it is you do.